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Partitioning of solar radiation in Arctic sea ice during melt season

Lu P., Cheng B., Leppäranta M., Li Z.

Oceanologia

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2018

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No. 60 (4)

464--477

EN

The partitioning of solar radiation in the Arctic sea ice during the melt season is investigated using a radiative transfer model containing three layers of melt pond, underlying sea ice, and ocean beneath ice. The wavelength distribution of the spectral solar irradiance clearly narrowed with increasing depth into ice, from 350-900 nm at the pond surface to 400-600 nm in the ocean beneath. In contrast, the net spectral irradiance is quite uniform. The absorbed solar energy is sensitive to both pond depth (Hp) and the underlying ice thickness (Hi). The solar energy absorbed by the melt pond (Ψp) is proportional only to Hp. However, the solar energy absorbed by the underlying ice (Ψi) is more complicated due to the counteracting effects arising from the pond and ice to the energy absorption. In September, Ψp decreased by 10% from its August value, which is attributed to more components in the shortwave band (<530 nm) of the incident solar radiation in September relative to August. The absorption coefficient of the sea ice only enhances the absorbed energy in ice, while an increase in the ice scattering coefficient only enhances the absorbed energy in the melt pond, although the resulted changes in Ψp and Ψi are smaller than that in the albedo and transmittance. The energy absorption rate with depth depends strongly on the incident irradiance and ice scattering, but only weakly on pond depth. Our results are comparable to previous field measurements and numerical simulations. We conclude that the incident solar energy was largely absorbed by the melt pond rather than by the underlying sea ice.

2

Influence of landfast ice on the hydrography and circulation of the Baltic Sea coastal zone

Merkouriadi I., Leppäranta M.

Oceanologia

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2013

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No. 55 (1)

147-166

EN

The influence of landfast ice on hydrography and circulation is examined in Santala Bay, adjacent to the Hanko Peninsula, Gulf of Finland. Three-dimensional electromagnetic current meters and conductivity-temperature-depth (CTD) sensors were deployed in winters 1999-2000 and 2000-2001 during the Finnish-Japanese "Hanko 9012" experiment. In each winter, data collection started one month before the initial ice formation and lasted until one month after the ice had melted completely. Temperature and salinity are compared with long-term data from the Tvärminne Zoological Station, also located on the Hanko Peninsula. The water temperature was 2°C less than the long-term average. Ice formation and melting show up in the salinity evolution of the water body, which makes salinity a good indicator of ice formation and breakup in Santala Bay. The circulation under the ice became weaker by almost 1 cm s-1.

3

The progress in knowledge of physical oceanography of the Gulf of Finland: a review for 1997-2007

Soomere T., Myrberg K., Leppäranta M., Nekrasov A.

Oceanologia

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2008

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No. 50 (3)

287-362

EN

The main findings of studies of the physical oceanography of the Gulf of Finland (GoF) during 1997-2007 are reviewed. The aim is to discuss relevant updates published in international peer-reviewed research papers and monographs, bearing in mind that a comprehensive overview of the studies up to the mid-1990s is available (Alenius et al. 1998). We start the discussion with updates on the basic hydrographical and stratification conditions, and progress in the understanding of atmospheric forcing and air-sea interaction. Advances in the knowledge of basin-scale and mesoscale dynamics are summarised next. Progress in circulation and water exchange dynamics has been achieved mostly by means of numerical studies. While the basic properties of circulation patterns in the gulf have been known for a century, new characteristics and tools such as water age, renewal index, and high-resolution simulations have substantially enriched our knowledge of processes in the Gulf of Finland during the last decade. We present the first overview of both status and advances in optical studies in this area. Awareness in this discipline has been significantly improved as a result of in situ measurements. Our understanding of the short- and long-term behaviour of the sea level as well as knowledge of the properties of both naturally and anthropogenically induced surface waves have expanded considerably during these ten years. Developments in understanding the ice conditions of the Gulf of Finland complete the overview, together with a short discussion of the gulf's future, including the response to climate change. Suggestions for future work are outlined.